I think the biggest reason for most of what I have read is because they bought them new and didn't give them time to adjust to their new surroundings before chopping and putting into a new pot, because they want it to look the way they want in the fancy new pot they bought for that tree right now.
Else this thread would read I just got this new plant and it dropped all its leaves shortly after bringing it home.
Every time you repot it will drop almost all its leaves...lol...the cycle repeats.
I leave mine outside in S. Texas year round in one spot, just moving it to rotate it till the temps get to about 40. At that point for me it will turn all the leaves black overnight and drop them.